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This is just a simple rule of probability. The likelihood that an event, composed of two or more independent events, will occur is equal to the mathematical product of the likelihood that each event will occur independently.
Review the bulleted definition above to see that it also makes biological sense. The likelihood that a given genotype will form is equal to the probability of picking first one then the second of the two alleles necessary to form it. And, the probability of picking an allele directly reflects its relative commonness (frequency) in the parental population .
Finally, note that this likelihood also represents the frequency with which we expect to observe the genotype in the offspring generation. This makes sense because how frequently we expect to see the genotype in the offspring generation is the direct result of how likely it is to form. Genotypes that have a low probability of forming, because the alleles that comprise them are relatively rare in the parental generation, will appear infrequently in the offspring generation.
There is one more thing to consider when determining the probability that given genotype will appear in the next generation; that is the number of ways a particular genotype can form.
Remember that the likelihood a particular genotype will form is equal to the probability of picking first one then the second of the two alleles necessary to form it. There is only one way to form a homozygote; both parents must donate the same allele. For example, the AA genotype can only form if, when one parent donates an A , the other does as well. Heterozygotes, in contrast, can form in two ways; 'parent one' can donate an A and 'parent two' an a or 'parent one' can donate an a and 'parent two' an A .
The fact that there are two routes to heterozygote formation must be taken into consideration when calculating the likelihood that the heterozygous genotype will occur in the offspring generation. To account for this, you must multiply the probability that a heterozygote will form by two, i.e. multiply the value described in the bulleted point above by two.
You should now have the tools to answer the question posed at the start of this module:
To answer the question titling this section and the module, let’s start by asking what you need to know. Then let’s figure out how you can get that information. To do this, review the question, the information provided in previous sections and develop a table similar to that below.
What do I need to know to answer this question? | How do I get this information? |
Check your outline above by working through the example below.
Review Figure 1. Imagine that these parents mate randomly. What genotypes could their offspring exhibit? Put another way, what allele combinations could we see in the offspring generation if every parent has an equal chance of reproducing and thus contributing an allele to the next generation?
Since two alleles exist for this locus in this population, A and a , three possible genotypes can occur in the offspring of these parents. These three genotypes are:
Recall that the last can form in two ways: Aa or aA .
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